The (1-53)H:(54-104) complex, an ordered complex formed by non-covalent interactions of the two fragments of cytochrome c formed by a limited trypsin digestion of the parent complex (1-65)H:(1-104), has been used in fragment exchange experiments to determine the activation free energy for unfolding of both oxidized and reduced forms of the complex. The slower rate of exchange of the ferrous form is consistent with its lack of susceptibility to trypsin digestion. The data support the concept of a dynamic equilibrium of folding and unfolding of cytochrome c under physiological conditions. BIBLIOGRAPHIC REFERENCE: Hantgan, R.R. and Taniuchi, H,: Formation of a Biologically Active, Ordered Complex from Two Overlapping Fragments of Cytochrome C. J. Biol. Chem. 252, 1367-1374, 1977.